Microstructure and Wear Properties of a Low Carbon Bainitic Steel on Plasma Nitriding at Different N2-H2 Gas Mixtures

For the first time, the influence of gas mixture on first damage resistance of a plasma nitrided DIN 18MnCrSiMo6-4 bainitic steel was investigated. Samples were nitrided at 500 °C with three different N2-H2 gas mixtures, containing 5, 24, and 76 vol.% N2. Samples were characterized concerning the resulting roughness, microstructure, compound layer's phase composition, residual stresses in the diffusion zone, and surface hardness. Tribological ball-on-flat tests were carried out in reciprocal mode using zirconia as ball material for friction coefficient and the compound layer resistance until the first damage. The test results were evaluated statistically by analysis of variance (ANOVA). As the amount of nitrogen in the gas mixture decreases, the ε-Fe2-3(C)N content in the compound layer decreases. A γ'-Fe4N monophasic compound layer was achieved at 5 vol.% N2 gas mixture. The diffusion zone as expected presented compressive residual stresses with the highest values near the surface. In the tribological tests, better results were obtained for 5 and 24 vol.% N2 in the gas mixture as higher amounts of γ'-Fe4N were formed. The 76 vol.% N2 gas mixture led to a brittle behavior, due to the biphasic compound layer (γ'-Fe4N and ε-Fe2-3(C)N) with a predominant content of ε-Fe2-3(C)N.